1. Field of the Invention
The present invention relates to a hearing aid of the type having two hearing aid apparatuses worn at the head, arranged in the region of the two ears in order to supply audio sound, with sound emitters being routed from the hearing aid apparatuses into the ear canals, and having a connection that transmits signals between the two hearing aid apparatuses.
2. Description of the Prior Art
The capacity to concentrate on a speaker in a loud environment and to suppress other sources of disturbing sound is based largely on binaural (two-eared) hearing. Patients suffering from hearing loss should thus be provided as far as possible with hearing aids at both ears. For hearing-impaired persons with asymmetrical hearing loss, hearing aids are offered whereby a microphone is attached in the vicinity of one ear, while the other ear is supplied with the signals picked up at the hearing-impaired ear and amplified, in addition to the standard sound. Connections from one ear to the other are required for this purpose. In constructing hearing aids in the form of hearing-aid glasses, leads are routed via the eyeglass frame (German Gebrauchsmuster 76 15 686). For this purpose, however, it is necessary that the leads run via the eyeglass hinge. These bending points are a source of defects, because the leads are bent during use, and/or contacts susceptible to disturbance must be employed. It is, however, burdensome for the hearing-impaired person to have electrical leads running on another path, from one ear over the head to the other ear. In addition, these electrical line connections between the hearing aid of one ear and the hearing aid of the other ear are highly susceptible to interference from electromagnetic fields produced by transistors employed in the hearing aid circuitry.
In order to be able to avoid an electrical connecting line between a microphone on one side of the head and an earphone on the other side of a person with asymmetrical hearing loss, it is known from German PS 23 60 342 to allocate to each ear a hearing aid assembly to be worn at the head; in one assembly the earphone, and in the other assembly the microphone, is replaced by an induction coil. This arrangement, which is very sensitive to disturbance from electromagnetic fields, can be realized through the use of two hearing aid apparatuses to be worn behind the ear, one of which is modified by exchanging the earphone for an induction coil. Another possibility is to use hearing-aid glasses in which a hearing aid is housed in each side piece. In this arrangement as well, on one side the earphone is exchanged for the induction coil, and on the other side the hearing aid is switched over for reception via the induction coil, or is hardwired with the coil instead of with the microphone.
For improving the hearing of hearing-impaired persons having asymmetrical hearing loss, the CROS (Contralateral Routing of Signal) method (the routing of a sound signal from one side of the head over to the other) can be used. To improve intelligibility when the hearing-impaired person is addressed from the side at which hearing is particularly impaired, the sound is thus picked up by the microphone of the hearing aid at the deaf ear and is supplied to the ear with good hearing, which remains open, at the other side of the head. In this way, the portion of the high frequencies that is attenuated due to the sonic effect of the head can be supplied to the ear with good hearing, and this portion is superimposed on the sound that the ear receives immediately from the sound field due to the open auditory passage. In this way, a unilaterally deaf person can, for example, better understand his or her neighbor sitting on his or her deaf side during round-table discussions.
In general, the CROS method is primarily recommended for those hearing-impaired persons whose hearing loss is limited to the upper speech frequency region. In supplying such a person with a hearing aid, the sound attenuation effect of the head is then used to acoustically separate the microphone and the earphone from one another. The amplification of the hearing aid at high frequencies can then be set higher than usual without the disturbance of acoustic feedback. In place of a snugly seated fitting earpiece, the auditory passage can remain outwardly open. The sound that the earphone produces is supplied to this auditory passage via an acoustic line.
In addition, the BICROS method is known for persons with asymmetrical hearing loss who also have a relatively great amount of hearing loss in the better ear. Here two microphones are used, located in the vicinity of each ear inlet, and connected to a single amplifier. The earphone emits the sound to the auditory passage of the better-hearing ear, connected to the fitting earpiece. The hearing aid suited for the BICROS method is also provided with a selector switch, by means of which either the microphones can be connected to the input of the amplifier together, or each can be connected to this input in itself. In this way, the hearing-impaired person can not only exploit the BICROS method but also can switch the hearing aid like a normal hearing aid, if he or she places the microphone located at the worse-hearing side out of operation.
Finally, signal processing systems are known for binaural hearing aids ("Psychoacoustics, Speech and Hearing Aids," B. Kollmeier, Verlag World Scientific 1995, page 246), to which the sound signals or characteristics obtained therefrom are supplied from hearing aid apparatuses worn at the right and left ear. In these systems, by means of comparison, correlation, etc., parameters are calculated that then influence the further signal processing in one or the other hearing aid apparatus, or in both of them. The suppression of disturbing noise from particular spatial directions can thereby be improved, e.g. by means of automatic adaptation of filter banks to the current disturbing sound scenario, or reverberation (echoes) can be reduced. These systems require bidirectional communication between the hearing aids worn at both sides of the head.